Advances in microelectronics technology tend to produce circuit chips which occupy less space while performing more functions. As the number of electrical interconnections between the chip and external circuitry required for the circuit in the chip to communicate with the outside world increases, the physical size of each such interconnection must decrease. In order to provide electrical communication between the circuit chip and external circuitry, circuit chips are usually contained within a housing or package which supports interconnection leads, pads, lands, etc., on one or more of its external surfaces. In order to reduce overall lead length from chip to external circuitry and to provide adequate spacing between input/output terminals on the package, high pin count devices are usually contained in packages with input/output terminals arranged in a grid pattern on one face of the package. The terminals may be in the form of pins extending from the package (usually described as a pin grid array or PGA) or contact pads or lands on the surface of the package (usually described as a land grid array or LGA). To physically attach the package to a substrate and provide electrical communication between its terminal lands and similar interconnect pads on the surface of a substrate such as a circuit board or the like, a small drop or ball of solder or the like is secured to each terminal land on the device package. Since the solder drop forms a ball-like protrusion extending from the terminal land, such devices are ordinarily described as ball grid array (BGA) device packages. Conversely, solder bumps may be formed on the interconnect pads or lands on the substrate which then mate with the lands on the LGA.
By aligning the lands or solder balls with corresponding interconnect pads or solder bumps on the surface of the board, grid array device packages may be electrically connected and structurally fastened to the circuit board surface without the need for pins or holes in the circuit board. This permits mounting circuit devices on both sides of the circuit board as well as increasing the density of device packages on the board. While use of less space is a great benefit, directly attaching the package to the circuit board creates some problems, particularly if the package must later be removed. Sockets attached to the surface of the board to receive the device packages and hold them in appropriate electrical contact while permitting removal and replacement are therefore desireable in many applications.
It is also very difficult to test BGA packages because of the difficulty in repeatedly making reliable electrical contact with the solder ball which forms the terminal without damaging the terminal ball. Since the terminal ball is solder, contact pins forced against the terminal tend to deform the ball or become bonded thereto by migration of the solder around the contact pin.
Reduction in size combined with more circuit functions necessarily results in generation of more heat in less space and thus less ability to dissipate that heat. As a result, it is frequently necessary to use heat sinks attached to the device packages to dissipate heat generated by the circuit chip. Unfortunately, prior art sockets (both for end use and for test purposes) generally employ a hinged cover or lid to hold the package in place. Such covers, however, interfere with and thus preclude mounting of device packages with attached heat sinks and, because of the stresses applied, may warp the socket base and thus damage the connections between the mounting socket and the substrate.